Harmonicity and its resulting temporal regularity are cues that might be used for sound source determination. Such complex stimuli often generate the perception of pitch. This project addresses issues concerned with temporal processing of the pitch of complex sounds, primarily rippled noises. Rippled noise generates complex pitch perceptions, exactly like those produced by many other complex stimuli. Complex pitch is one possible cue for sound source determination. Previous human psychophysical work suggests that the processing of iterated rippled noise can best be explained by temporal mechanisms such as might be revealed by forms of auto-correlation. Thus, a major focus of this project is on temporal processes such as auto-correlation. This project uses a multi-disciplinary approach based on human psychophysics, animal psychophysics, animal neurophysiology, and modeling to gain insights into the temporal mechanisms underlying pitch perception. A series of human psychosocial experiments is designed to determine how well auto-correlation and its neural analog, summary correlograms, can account for the pitch and pitch strength of complex sounds such as iterated rippled noise. In the animal psychophysical experiments, the perceptual capabilities of the chinchilla for processing the same types of rippled noise stimuli used in human psychosocial experiments will be evaluated. One goal is to determine if the mechanisms responsible for the perception of complex pitch in human subjects are unique or special to the human nervous system or whether these are common neural mechanisms that exist across mammals. In neurophysiological experiments, the neural representations of the temporal and spectral characteristics of the same rippled noises used in the human and chinchilla psychophysical experiments will be explored. Most of the physiological work will involve studies of single neurons in the chinchilla cochlear nucleus. The goal is to understand how information about harmonicity and its resulting temporal regularity and pitch are represented and processed in the auditory system.